Spread spectrum receiver having phase shifter for effecting phase synchronization of two convolvers

ABSTRACT

A phase shifter is disclosed which performs a desired phase shift by distributing an input signal to two signals whose phases are different by 90° from each other which signals are added to each other after having been weighted with a sine and a cosine function signal, the sine and the cosine function signal being able to be set in a digital manner.

FIELD OF THE INVENTION

This invention relates to a phase shifter and in particular to animprovement in the phase shifter which can be used in practice in a highfrequency circuit such as a receiver for spread spectrum communication.

BACKGROUND OF THE INVENTION

FIGS. 4a and 4b illustrate prior art phase shifters. FIG. 4a indicates apassive type phase shifter consisting of resistances R and R/2, acapacitance C and an inductance L while FIG. 4b indicates an active typephase shifter consisting of an operational amplifier AMP, resistances Rand R' and a capacitance C.

In the phase shifter indicated in FIG. 4a, if the inductance L and thecapacitance C are selected so as to satisfy: ##EQU1## the frequencycharacteristics thereof are represented by curves as indicated in FIG.5. As it can be clearly seen from the figure, the amplitude is constantand the phase is given by: ##EQU2##

Also in the phase shifter indicated in FIG. 4b, in the same way, thefrequency characteristics that the amplitude is constant and the phaseis given by Equation 3 are obtained by selecting the resistance R andthe capacitance C so as to satisfy: ##EQU3##

However, in the case when the phase is controlled in a wide range (atleast from 0° to 360°), the phase shifters indicated in FIGS. 4a and 4bare not suitable because it is necessary to connect at least 2 of thecircuits indicated in FIGS. 4a and 4b in cascade as it is clearly seenfrom FIG. 5 and both the values L and C should be variable for thecircuit indicated in FIG. 4a as it can be understood from Equations (1)and (2). Further, in order to keep the amplitude constant, the relationbetween L and C should be kept constant and it is difficult to controlthe phase from the exterior. Next, it makes the phase shifter indicatedin FIG. 4b unsuitable for use in a high frequency circuit that anoperational amplifier AMP is required therefor. That is, it is not easyto realize a high input impedance, a low output impedance, a high gain,etc. which are necessary conditions for the operational amplifier in ahigh frequency. In prior art, phase shifters other than those indicatedin FIGS. 4a and 4b in order to keep the amplitude constant both theinductance and the capacitance should varied and sometimes the amplitudecan not be kept constant. Thus it is unsuitable for the phase controlfrom the exterior and in particular for automatic control.

OBJECT OF THE INVENTION

Consequently the object of this invention is to provide a phase shifterwhich can vary the phase basically in the infinite region; has frequencycharacteristics that the amplitude is constant and only the phasevaries; can vary easily the phase; and is suitable for the phase controlfrom the exterior.

SUMMARY OF THE INVENTION

In order to meet the foregoing object, a spread-spectrum receiveraccording to the invention includes: a correlator which includes firstand second convolvers each having first and second inputs, the firstinputs each being supplied with a common spread-spectrum receivedsignal; a first multiplier for applying a first reference signalproduced by multiplication of a first CW signal and a first PN code tothe second input of the first convolver; a signal distributor suppliedwith the first CW signal for distributing the first CW signal into firstand second signals which are different in phase by 90 degrees; afunction signal generator responsive to a predetermined digital signalfor generating a sine function signal and a cosine function signal; aweighting arrangement for weighting the first and second signals by thesine function signal and the cosine function signal, respectively; aphase shifter which includes an adder for adding the weighted first andsecond signals from the weighting arrangement to generate a second CWsignal which has the same frequency as and a predetermined phasedifference from the first CW signal; a second multiplier for applying asecond reference signal produced by multiplication of the second CWsignal and a second PN code to the second input of the second convolver;and a demodulator for demodulating data by multiplying outputs of bothof the convolvers.

In the phase shifter according to this invention, the function signalgenerating means described above consists of memories and D/A convertersand sends the digital signal outputted by the memories by giving thememory address data corresponding to the desired phase angle to beshifted after having been D/A-converted in order to phase-shift theinput signal by a desired phase angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a phase shifter circuit;

FIG. 2 is a block diagram of a spread-spectrum receiver embodying thepresent invention, one component of which is the phase shifter circuitof FIG. 1;

FIGS. 3A and 3B show waveforms for certain signals in the receiver ofFIG. 2;

FIGS. 4A and 4B are schematic diagrams of conventional phase shiftercircuits; and

FIG. 5 is a graph showing a characteristic of the phase shifter of FIG.4A.

DETAILED DESCRIPTION

Hereinbelow this invention will be explained by referring to anembodiment indicated in the drawings in which FIG. 1 illustrate anembodiment of the phase shifter according to this invention.

In the figure, reference numeral 1 is a distributer; 2 and 2' areweighting means, e.g. multipliers; 3 is an adder; 4 and 4' are memories;and 5 and 5' are D/A converting circuits.

An input signal a to be phase-shifted is given to the distributer 1 anddistributed to a first and a second signal b and c respectively whosephases are different by 90° from each other which are sent to themultipliers 2 and 2'.

On the other hand, sine and cosine function data SINφ and COSφ arestored in the memories 4 and 4' respectively and digital signals j and krepresenting desired sine and cosine functions are read-out by addressdata i.

These digital signals are D/A-converted by the D/A converting circuits 5and 5'. The sine and the cosine function signal h and g obtained as theresult are given to one input of the multipliers 2 and 2' and multipliedby the first and the second signal given to the other input thereof. Theoutputs of the multipliers are added to each other by the adder 3 and anoutput signal f which is identical to the input signal a whose phase isshifted by φ is obtained.

The operation of the phase shifter described above will be explainedmore concretely. The input signal a which is represented e.g. by COSω₀ tis inputted in the distributer 1 and distributed to the signals b and cwhose phases are different by 90° from each other. That is, the phase ofthe signal c is represented by COSω₀ t and that of the signal b by SINω₀t. The signal b is multiplied by a DC current g corresponding to SINφ inthe multiplier 2 which outputs a signal d represented by SINω₀ t·SINφ.On the other hand, the signal c is multiplied by another DC current hcorresponding to COSφ in the multiplier 2' which outputs a signal erepresented by COSω₀ t·COSφ. These signals d and e are added by theadder 3 to each other and the output signal f whose phase is shifted byφ with respect to the input signal a can be obtained.

This operation can be represented by equations as follows:

    ______________________________________                                        input     signal  -a    COSω.sub.0 t                                              signal  -b    SINω.sub.0 t                                              signal  -c    COSω.sub.0 t                                              signal  -d    SINω.sub.0 t · SINφ                          signal  -e    COSω.sub.0 t · COS φ               output    signal  .sub.-f                                                                             COS(ω.sub.0 t - φ)                                    signal  -g    SINφ                                                        signal  -h    COSφ                                              ______________________________________                                    

When the signals d and e are added by the adder 3 to each other, thisoperation can be given by: ##EQU4## The output signal f whose phase isdelayed by φ with respect to the input signal a is thus obtained.

In order to obtain the signals g and h, address data i corresponding tothe phase φ are given to the memories 4 and 4' respectively. Data ofSINφ are stored at the address corresponding to the phase φ in thememory 4 and data of COSφ are stored at the address corresponding to thephase φ in the memory 4' which output the data i of SINφ and the data kof COSφ respectively. These digital data of SINφ are converted into theDC current g corresponding to SINφ by the D/A converting circuit 5 andthe digital data of COSφ are converted into the DC current hcorresponding to COSφ by the D/A converting circuit 5'.

Further, it is sufficient for the DC currents g and h to satisfyrelatively the relation between SINφ and COSφ and it is unnecessary toadjust the absolute values thereof. It is thus possible for the phaseshifter according to this invention to make adjustment unnecessary.

Furthermore, even if there are disposed no memories separately, thephase shifter is so constructed that SINφ and COSφ are calculated by aCPU starting from φ, and they are given directly to the D/A convertingcircuits.

FIG. 2 shows a constructional example of a receiver for spread spectrumcommunication using the phase shifter indicated in FIG. 1 in whichreference numerals 15 and 16 are convolvers; 17 and 18 are multipliers;19 is a phase shifter having the construction identical to thatindicated in FIG. 1; 20 and 21 are amplifiers; 22 is a multiplier; 23 isa low pass filter; 24 is a switch; 25 is a CPU for the phase control;and 26 is an A/D converting circuit.

When the switch 24 is turned over on the contact I₁ side by the CPU 25,a received spread spectrum communication signal S is applied to oneinput of each of the convolvers 15 and 16 and a first and a secondreference signal R_(f1) and R_(f2) are applied to the other inputsthereof.

A CW signal CW₁ having the same frequency as the RF carrier signal inthe spread spectrum communication signal S is given to one input of eachof the phase shifter 19 and the multiplier 17. The phase shifter 19shifts the phase of the CW signal CW₁ by 90° and supplies it to oneinput of the multiplier 18. PN codes PN₁ and PN₂ necessary fordemodulation are given to the other inputs of the multipliers 17 and 18respectively and the outputs of the multipliers 17 and 18 are the firstand the second reference signal R_(f1) and R_(f2) respectively.

The convolvers 15 and 16 correlate the spread spectrum communicationsignal S with the first and the second reference signal R_(f1) andR_(f2) respectively. The correlation outputs V_(c1) and V_(c2) areapplied to the multiplier 22 through the amplifiers 20 and 21respectively and the output of the multiplier is given to the low passfilter 23 so as to obtain the data demodulation signal V_(f).

It is explained how the data demodulation signal V_(f) can be obtainedstarting from the spread spectrum communication signal S received by theconstruction of the embodiment described above.

The received spread spectrum communication signal S can be representedby:

    S=Vd(t)=P.sub.1 (t)SIN(ω.sub.0 t)+A·P.sub.2 (t)COS(ω.sub.0 t)                                   (1)

where P₁ (t) and P₂ (t) are the first and the second PN coderespectively which are used at the modulation on the transmitter side; Arepresents data which are 1 and -1; and the signal S is equally given tothe two convolvers.

The first and the second reference signal R_(f1) and R_(f2) inputted inthe two convolvers can be represented by:

    R.sub.f1 =V.sub.r1 (t)=P.sub.1 (t)COS(ω.sub.0 t)     (2)

    R.sub.f2 =V.sub.r2 (t)=P.sub.2 (t)COS(ω.sub.0 t+θ) (3)

where θ indicates the phase shifted by the phase shifter; P₁ (t) and P₂(t) are the PN codes PN₁ and PN₂ used at the demodulation on thereceiver side which are mirror images (time inverted signal) of P₁ (t)and P₂ (t) used on the transmitter side.

The outputs V_(c1) and V_(c2) of the two convolvers are given by:

    V.sub.c1 (t)=CONV{Vd(t), V.sub.r1 (t)}                     (4)

    V.sub.c2 (t)=CONV{Vd(t), V.sub.r2 (t)}                     (5)

where CONV{V₁ (t), V₂ (t)} represents the convolution of the two inputsV₁ (t) and V₂ (t). Supposing that

    V.sub.1 (t)=COS(ω.sub.0 t)                           (6)

    V.sub.2 (t)=COS(ω.sub.0 t+θ)                   (7)

the output of the convolver CONV{V₁ (t), V₂ (t)} is given by:

    CONV{V.sub.1 (t), V.sub.2 (t)}=η·COS(2ω.sub.0 t+θ+φ)                                          (8)

where η is the convolver efficiency and φ is the additional phase whichis proper to the convolver. In this way, it can be understood thatvariations in phase θ of the one input appear at the output as they are.

Since the correlations between P₁ (t) and P₂ (t) and between P₂ (t) andP₁ (t) are small, the following approximation

    V.sub.c1 (t)≈CONV{P.sub.1 (t)SIN(ω.sub.0 t),P.sub.1 (t)COS(ω.sub.0 t)}                                  (9)

    V.sub.c2 (t)≈CONV{A·P.sub.2 (t)COS(ω.sub.0 t),P.sub.2 (t)COS(ω.sub.0 t+θ)}               (10)

do not give rise to great errors. Equations (9) and (10) are furthersolved as follows:

    V.sub.c1 (t)=η.sub.1 ·R.sub.1 (t)SIN(2ω.sub.0 t+φ.sub.1)                                            (11)

    V.sub.c2 (t)=η.sub.2 ·A·R.sub.2 (t)COS(2ω.sub.0 t+θ+φ.sub.2)                                    (12)

where R₁ (t) and R₂ (t) are convolutions between P₁ (t) and P₁ (t) andbetween P₂ (t) and P₂ (t); and φ₁ and φ₂ are additional phases which areproper to the two convolvers respectively.

The output V_(m) (t) after the multiplication of V_(c1) (t) and V_(c2)(t) is given by: ##EQU5##

In Equation (13) supposing that

    θ+φ.sub.2 =φ.sub.1 -π/2                   (14)

the following equation is obtained: ##EQU6##

Further, the demodulation signal V_(f) (t) obtained by making V_(m) (t)pass through a low pass filter is given by:

    V.sub.f (t)=η.sub.1 ·η.sub.2 ·A·R.sub.1 (t)·R.sub.2 (t)                                  (17)

FIG. 3 shows an example of V_(c1) (t), V_(c2) (t) and V_(f) (t) in thecase where φ₁ =φ₂ and from the figure and Equation (17), it can be seenthat the data demodulation is possible.

However, in Equation (14), if θ+φ₂ =φ₁, V_(f) (t)=0 is valid which meansthat the demodulation is not possible. As stated previously, φ₁ and φ₂are not always identical due to slight differences in electriccharacteristics, temperature characteristics and the length of wiring ofthe two convolvers.

Consequently, in this case, it can be seen that starting from Equation(14), the predetermined phase shift θ of the phase shifter may be givenby:

    θ=φ.sub.1 -π/2-φ.sub.2                    (16)

Therefore, in order to correct the phase shift in such a case in FIG. 2,the switch 24 is turned over to the contact I₂ side by the CPU 25 sothat the CW signal CW₁ is given to one of the input of the convolvers 15and 16 and further the first and the second PN code are switched over toDC bias voltages.

The input signal being represented by V_(d) (t), since

    Vd(t)=COS(ω.sub.0 t)                                 (18)

is valid and further the first and the second PN code are switched overto the DC bias voltages, the following relation is satisfied:

    P.sub.1 (t)=P.sub.2 (t)=1                                  (19)

Consequently, the convolution outputs V'_(c1) (t) and V'_(c2) (t) aregiven by:

    V'.sub.c1 (t)=η.sub.1 ·COS(2ω.sub.0 t+φ.sub.1) (20)

    V'.sub.c2 (t)=η.sub.2 ·COS(2ω.sub.0 t+φ.sub.2) (21)

Then the output V'_(m) (t) after the multiplication of V'_(c1) (t) andV'_(c2) (t) is represented by:

    V'm(t)=η.sub.1 ·η.sub.2 ·COS(2ω.sub.0 t+φ.sub.1)·COS(2ω.sub.0 t+φ.sub.2) (22)

Here, if

    θ+φ.sub.2 =φ.sub.1 -π/2                   (23)

Equation (22) can be transformed into:

    V'm(t)=η.sub.1 ·η.sub.2 ·COS(2ω.sub.0 t+φ.sub.1)·SIN(2ω.sub.0 t+φ.sub.1) (24)

and thus the output V'_(f) (t) of the low pass filter is given by:

    V'.sub.f (t)∝η.sub.1 ·η.sub.2 ·COS(φ.sub.1 -θ-φ.sub.2)           (25)

The output V'_(f) (t) stated above is transformed into a digital signalby the A/D converting circuit 26 which signal is inputted in the CPU 25.The CPU 25 outputs the address data corresponding to the phase θ whichis in the state represented by Equation (16), to the phase shifter 19responding to the digital signal and performs the phase control.

In this case, it is undetermined what values φ₁ and φ₂ have and ithappened that a prior art phase shifter could not cover the variableregion of this phase shift angle. On the contrary, the phase shifterhaving the construction according to this invention can shift the phasecontinuously. In addition, since the variable region of the phase shiftis not restricted, the phase shifter is suitable for a receiver forspread spectrum communication, etc.

As it is clearly seen from the explanation described above according tothis invention, excellent effects stated below can be obtained;

(i) it is possible to vary easily the phase merely by varying addressdata given to the memories;

(ii) it is possible to set arbitrarily the amount of variation in thephase by varying the resolution of the data of SINφ and COSφ stored inthe memories;

(iii) it is possible to vary the phase continuously by storing only datain a region 0°<φ<360° for the data of SINφ and COSφ stored in thememory;

(iv) it is possible to realize a phase shifter which keeps the amplitudeconstant and makes only phase vary in a wide frequency region and inparticular even in a high frequency region;

(v) owing to (i) and (iii) automatic control can be performed easily bymeans of a microcomputer, etc.;

(vi) owing to (i) to (iii) it is possible to obtain a phase shifter forwhich adjustment is unnecessary; and

(vii) it is possible to realize a phase shifter which is suitableparticularily for a high frequency circuit such as a receiver for spreadspectrum communication for which prior art phase shifters can not workefficiently.

What is claimed is:
 1. A spread-spectrum receiver, comprising:acorrelator which includes first and second convolvers each having firstand second inputs, said first inputs each being supplied with a commonspread-spectrum received signal; first multiplier means for applying afirst reference signal produced by multiplication of a first CW signaland a first PN code to said second input of said first convolver; signaldistributing means supplied with said first CW signal for distributingsaid first CW signal into first and second signals which are differentin phase by 90 degrees; function signal generating means responsive to apredetermined digital signal for generating a sine function signal and acosine function signal; weighting means for weighting said first andsecond signals by said sine function signal and said cosine functionsignal, respectively; adder means for adding said weighted first andsecond signals from said weighting means to generate a second CW signalwhich has the same frequency as and a predetermined phase differencefrom said first CW signal; second multiplier means for applying a secondreference signal produced by multiplication of said second CW signal anda second PN code to said second input of said second convolver; anddemodulating means for demodulating data by multiplying outputs of bothof said convolvers.
 2. A spread spectrum receiver according to claim 1,wherein said function signal generating means includes two memoriesrespectively storing sine function data and cosine function data, andtwo digital to analog converters which each convert a digital outputsignal from a respective one of said memories into a respective analogsignal, said analog signals being said sine function signal and saidcosine function signal.
 3. A spread spectrum receiver according to claim2, wherein said weighting means includes two further multipliers whicheach have a first input to which is applied a respective one of saidsine and cosine function signals from said digital to analog convertersand which each have a second input to which is applied a respective oneof said first and second signals.
 4. A spread spectrum receiveraccording to claim 1, including switch means for selectively supplyingone of said spread-spectrum received signal and said first CW signal tosaid first inputs of said first and second convolvers; and digitalcircuit means for causing said switch means to supply said first CWsignal to said convolvers and for simultaneously calculating saidpredetermined digital signal for said function signal generating meansin response to an output signal from said demodulating means.